1. Field of the Invention
The present invention relates to a semiconductor laser module, and more particularly to a semiconductor laser module having an external resonator.
2. Related Art
In a semiconductor laser module of the past, the most general technology for stabilizing the wavelength of the laser light output from the module was that of forming in an optical fiber one fiber grating having a diffraction grating at a wavelength at which the semiconductor laser can oscillate, and to form an external resonator by the rear surface (Hr surface) of the semiconductor laser element and the fiber grating formed on the optical fiber.
For example, in Journal of Lightwave Technology, Vol. 15, No. 8, August 1997, there is a description of technology in which an external resonator is formed by a 980 nm band laser element and a fiber grating formed in an optical fiber (the center reflected wavelength of which is 975 nm having a reflectivity of 2 to 4%), thereby stabilizing the wavelength of the laser light emitted from the semiconductor laser module to 975 nm.
The above-noted technology, intended for the purpose of stabilizing the wavelength of the laser light output from a semiconductor laser module, has the following problem.
Specifically, because there is a limit to the center oscillation wavelength of the semiconductor laser element which can be pulled in by a fiber grating, the farther mutually removed are the center reflection wavelength of the fiber grating and the center oscillation wavelength of the semiconductor laser element, the more difficult it is to pull in the laser light emitted from the semiconductor laser element to the wavelength of the fiber grating. In this case, therefore, there is the problem of additional oscillation at the stand-alone oscillation wavelength of the semiconductor laser element in addition to the oscillation at the wavelength established by the fiber grating, thereby causing the light output to have an unstable wavelength.
With further distance between the fiber grating reflection center wavelength and the center oscillation wavelength of the semiconductor laser element, it becomes absolutely impossible to pull in the laser light from the semiconductor laser element to the wavelength established by the fiber grating, resulting in the semiconductor laser element oscillating at its own center oscillation wavelength, with light being output from the semiconductor laser module at this stand-alone semiconductor laser element oscillation wavelength.
Technology intended for solving the above-noted problem is proposed in the Japanese Unexamined Patent Publication (KOKAI) No. 2000-131559 (hereinafter referred to as the first disclosure), which shows a laser diode module. The laser diode module of the first disclosure is one in which a laser diode light source with an internal laser diode chip is optically coupled to an optical fiber. The optical fiber is a grating fiber, the reflection center wavelength of which is established so as to be 2 to 10 nm shorter than the oscillation center wavelength of the laser diode chip at 25° C.
More specifically, in the laser diode module of the first disclosure, a chip having oscillation center wavelengths of 968 nm at 0° C., 975 nm at 25° C. and 981 nm at 40° C. is used as the laser diode chip, and the center reflection center wavelength of the optical fiber grating is set to be 970 nm. In a laser diode module having this configuration, according to the first disclosure, over a temperature variation of ±25° C. with respect to the normal ambient temperature of 25° C., it is possible to lock the laser diode oscillation center wavelength to the fiber grating reflection center wavelength of 970 nm.
With the above-noted technology of the first disclosure, however, when fabricating a semiconductor laser module because it is necessary to achieve somewhat of a coincidence between the semiconductor laser element oscillation center wavelength and the reflection center wavelength of the fiber grating, it is necessary to select the semiconductor laser element to be used, this being disadvantageous from the standpoint of fabricating a low-cost semiconductor laser module.
Given the above, in order to stabilize the wavelength of the laser light from a semiconductor laser module, and also solve the above-noted problem with the technology of the first disclosure, the oscillation center wavelength range of the semiconductor laser module that can be pulled into the wavelength of the fiber grating can be broadened. The oscillator center wavelength of a semiconductor laser element that can be pulled into the wavelength of a fiber grating is generally established by the relationship between the front-surface reflectivity of the semiconductor laser element and the fiber grating reflectivity, and it is most effective to make the fiber grating reflectivity high.
However, if the reflectivity of the fiber grating is merely made high, the coherency of the reflected light from the fiber grating as seen from the semiconductor laser element becomes high. Using this approach, therefore, although it is possible to broaden the range of semiconductor laser element oscillation center wavelength that can be pulled in to the wavelength of a fiber grating, problems arise such as a kink occurring in the light output characteristics after the module is fabricated.
Technology intended to stabilize the light output characteristics is also proposed in the Japanese Unexamined Patent Publication (KOKAI) No. H10-293234 (referred to hereinafter as the second disclosure). In a semiconductor laser module described in the second disclosure, fiber gratings which reflect light of mutually different wavelengths is formed in an optical fiber. These two fiber gratings are formed within the package that holds the semiconductor laser chip, or in a location at which a coupling means that couples the package to an optical fiber is disposed.
Although with a semiconductor laser module as described in the second disclosure, it is possible to stabilize the light output characteristics, because the reflection center wavelengths of the two fiber gratings forming in the optical fiber mutually differ, there is the problem of oscillation in two modes having different wavelengths.
In the Japanese Unexamined Patent Publication (KOKAI) No. 2000-194023 (hereinafter referred to as the third disclosure), there is a description of a semiconductor laser module in which two fiber gratings having mutually different reflection wavelengths, wherein the reflection center wavelengths of these two fiber gratings are set as to be above and below a prescribed wavelength of the light emitted from a semiconductor laser element.
Because the semiconductor laser module of the third disclosure is configured as noted above, in the case in which the oscillation center wavelength of the semiconductor laser module is shorter than the reflection center wavelengths of the two fiber gratings, oscillation occurs at only the wavelength established by the fiber grating having the shorter reflection center wavelength. Further in the case in which the oscillation center wavelength of the semiconductor laser module is between two reflection center wavelengths of the two fiber gratings, oscillations occur at the two wavelength established by the two fiber grating. Similarly, in the case in which the oscillation center wavelength of the semiconductor laser module is longer than the reflection center wavelengths of the two fiber gratings, oscillation occurs at only the wavelength established by the fiber grating having the longer reflection center wavelength. Thus, with a semiconductor laser module according to the third disclosure, there is the problem that the wavelength of the laser light obtained from the optical fiber will differ, depending upon the oscillation center wavelength of the semiconductor laser module.
Accordingly, it is an object of the present invention to provide a semiconductor laser module capable not only of stabilizing the wavelength of the laser light even if there is variation in the oscillation center wavelength of the semiconductor laser element, but also of limiting variation in the light output characteristics.